Add Particle identifier.

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Related to #231, Particle and ParticleIO ...

@kathy-phet suggested adding an id to each Particle, so that individual particles can be tracked across state snapshots.

This id could probably be the order in which it was created (0-999 in the first 3 screens, 0-199 in the Diffusion screen). But I don't see any easy/trivial way to add this. Complicating things is the fact that temporary Particles are created for other purposes, like creating icons, Sprites, etc.

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@kathy-phet noted that specific particles leave through the container's lid. So we can't use rely on their index matching the order that they were created. And we cannot (for example) expect that particles will have the same index between samples. So we'll need some other scheme for the id.

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This is proving to be complicated and costly. So I'm going to push back on this request. @arouinfar @kathy-phet Why do particles need unique identifiers for PhET-iO?

In 4/31 and 5/6 data fluency meetings, there were no requests to track the attributes of specific particles over time. In 5/13/24 data fluency meeting for #236, I specifically asked Bill Finzer (Concord) about whether there was a need to inspect individual particles. He responded with a strong "no". We should be comparing snapshots of the particle system, not individual particles.

@kathy-phet pointed to how her brother uses this sim to teach Brownian motion. For example, pumping 1 light particle into a contaner of heavy particles, then observing the motion of the light particle. While that's an interesting use of the sim, it's not what the sim was designed to address. PhET is unlikely to prioritize/encourage building a PhET-iO wrapper for such a use case, so it doesn't seem like a reason to add particle identifiers.

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Design meeting 5/20/24 @arouinfar @kathy-phet @Nancy-Salpepi @matthew-blackman @pixelzoom

We decided not to add a particle identifier, because tracking individual particles is not aligned with the goals of this sim.

Closing.